Add diffusion as test case

* Pythran performs well on the pure python code but not on the numpy version.
Merge branch 'yuancheng-yuanchengBranch'
Fix value_type for numpy_expr

* value_type of : empty_like(ndarray<long, 2>) should be ndarray<long,1> instead of long
Add dtype and T attributes to numpy array
Fix shape for gslice

* case of gslice without all its dimension named in the slice
* ex: a.shape = (2,3,4)
    b = a[:,1]
    print b.shape
gslice are built from const array
Rework None implementation

Previous implementation had typing information spread over several files
(bad locality) and did not handle using member function on the none
type.
Add missing const qualifier for string::operator==
Modification in indexed_array implementation
Improve gslice interface

* operator[]
* operator+=
* operator/=
* improve at performance
Fix islice

* avoid dangling reference (same issue than in imap b45f10ad2a28bd4fec027bd0a848304fb1d34663)
keep cpp files when compilation failed

* needed and really usefull for debug
export sliced string to python
Handle case of empty string
Add tests

* Many test from rosetta and for numpy
Fix assignment of gsliced_array
Use a CFG analysis to insert Return None in the missing places

* This fixes Issue #77
* The CFG does not take into account interprocedural exceptions
Make test_random more robust

Increase the number of sampling when testing random generator
implementation, to avoid false negative.

Execution time is still reasonable compared to compile time ;-)
Workaround clang++ list constructor bad disambiguification

clang++ is not capable of resolving against the right constructor for

    core::list<empty_list>({ proxy::list{}() })

so help it a little.
Merge pull request #128 from pbrunet/master

Improve numpy array + bugfixes

- Perf&features improvements for array slicing
- Handle case of empty string + sliced string export to python
- Indexed_array
- Add dtype and T attributes to numpy array
- Many tests from rosetta and for numpy